Shopify is just the next closed-source iteration of WordPress and Magento. WordPress has one of the largest platforms and they've taken over (and open-sourced) blogging, content management, learning management systems, and ecommerce. They and Magento are better examples of a platform than Shopify.
In the long-term, "platforms" like Shopify lose because they don't have nearly enough developers. They have ~3000 "apps" listed in their app store: https://apps.shopify.com/browse/all?pricing=all&requirements...
WooCommerce has 285 plugins, and WordPress has 54,000+ plugins. Magento has ~5000 plugins.
It would be cool if Stratechery took a look at free/open source on the platform/aggregator spectrum.